This invention relates to an artificial reef unit which has a high durability in the marine environment and is capable of increasing the productivity and harvestability of fishes.
Recently, attempts have been made by fisheries and propagating and/or cultivating places of fish using artificial reefs. In order to increase the productivity and harvestability of fish, many investigations on suitable structures of artificial reefs have been made from a viewpoint of the ecology of fish.
With respect to the conditions under which fish are attracted to reefs, there can be mentioned the following:
(1) Seaweed and shellfish adhere to the reef and then, small fish come together to take such seaweed and shellfish as bait.
(2) Middle size fish and large fish are attracted to the reef for feeding on the above-mentioned small fish as bait.
(3) Fish are attracted to the reef to use it as shelter from the attack of foreign enemies or as a resting place.
(4) Where a reef of a large-size, three-dimensional structure is present under the tidal currents, the nutrient-rich deep sea water is conveyed to the surface layer of the sea by the action of gushing, rising currents to promote propagation of plankton, thus providing a good bait place, to which fish are attracted; and
(5) The tidal current causes the reef to generate a whirlpool effect behind it, and plankton is made to gather under the whirlpool.
Taking into consideration the above-mentioned conditions, various structures of artificial reefs have been devised.
Representative examples of conventionally proposed artificial reefs include those built of scraps or discarded objects such as old ships, old cars, wooden frames, cracked stones, clay pipes, etc. However, with these materials, it is difficult to assemble or construct the reef into a desired shape or structure. In addition, it is not easy to collect necessary quantities of such scraps.
A further proposed approach is an artificial reef comprising a polyhedral concrete block. However, there are such disadvantages that because of its heavy weight this type of artificial reef is difficult to handle and usually needs a boat with a crane mounted thereon for installing it in a desired location.
On the other hand, there are known artificial reefs comprising a plurality of reef units each comprising a hollow cylindrical structure formed by spirally winding elongated fiber glass-reinforced plastics (see Japanese Patent Application Publications Nos. 830/1979 and 24953/1979 and Japanese Utility Model Registration Application Publication No. 8536/1980). Fiber glass-reinforced plastics are small in specific gravity and can be easily shaped into a desired structure and, hence, they are advantageously employable as materials for producing a light-weight, large-size artificial reef. As a result of further investigations on the actual performance of this type of reef by the present inventors, however, it has been revealed that a hollow structure formed by spirally winding elongated fiber glass-reinfored plastic alone, does not sufficiently perform as an excellent artificial reef unit. In other words, unless other specific conditions are satisfied, such a hollow structure formed by spirally winding elongated fiber glass-reinforced plastics will break easily under certain marine conditions, that it cannot be used for a long period of time and, in addition, the fish-gathering effect or fish harvestability is insufficient.
Illustratively stated, in some areas in which an artificial reef is installed, there are strong tidal currents and large changes in direction of tidal currents which cannot be expected by observation from the sea surface and, under some atmospheric conditions, there are generated extremely strong forces of waves. Due to the above phenomena, the artificial reef tends to break. In the case of a large-size artificial reef comprising a hollow structure made of elongated fiber glass-reinforced plastics spirally wound, the hollow structure located at the lowest position of the assembly of hollow structures first undergoes deformation and destruction, followed by sequential destruction of the other hollow structures.
With respect to the fish-gathering effect, it has been found that the open windows alone in the peripheral framework of the hollow structure made of elongated fiber glass-reinforced plastics spirally wound are not always effective for gathering various kinds of fishes. According to the shape and structure of the artificial reef, there are caused changes in the situation, direction and strength of the tidal currents as well as the current volume passing through the hollow structure, thereby affecting the fish-gathering effect. Further, it has been found that the shape and structure of the hollow structure have a great influence on the sheltering effect, restfulness and egg-laying effect for fish.
With a view to eliminating the drawbacks of the conventional artificial reef unit comprising a hollow structure made of elongated fiber glass-reinforced plastics spirally wound and to developing a novel artificial reef unit of such a type structure which is durable in the marine environment and extremely effective for gathering fish, the present inventors have made intensive studies. As a result, it has been found that an artificial reef unit comprising a hollow structure with its peripheral framework having lattice pattern windows arranged in specific relationships is extremely effective for the purposes of the present invention.